The Russians are the first brother-sister professional tennis twosome to have both earned the No. 1 ranking. The feat comes at a strange time, as Safina, who has never won a Grand Slam event, replaces Serena Williams, winner of two out of the last three such tournaments on the WTA tour. Professional athletes in other sports, including fencing and tae kwon do, rival the feat of Safin and Safina, and highlight the influence of genetics and environment in a family’s athleticism.

According to The Associated Press, Safina officially claimed the No. 1 women’s ranking on Monday, but still feels she has much work to do. She discussed her brother’s success and how it pushes her to work harder. “He has two Grand Slams. He’s still much better than me, so I have to catch him.”

Venus Williams has been called one of America’s most powerful women. Read more about the Compton native’s rise to tennis stardom.

“Their spirits are pretty much identical … as parents we put so much effort and time so that they would grow together,” Islanova said.

The article also claims that “brother-sister combinations at the highest level are rarer than the same-gender variety,” such as another famous tennis duo, the Williams sisters, Venus and Serena. However, this could change, as athletic opportunities for women continue to develop around the world, the Times reported.

Last April, NPR reported on the overachieving Lopez family. Siblings Diana, Mark and Steven all qualified for the 2008 Beijing Olympics in tae kwon do.

Last year, Olympic sibling athletes such as fencers Keeth and Erinn Smart, rowers Cameron and Tyler Winklevoss, and tennis-playing twins Bob and Mike Bryan all made headlines. The Smarts, in particular, faced difficult setbacks en route to Olympic stardom; their father passed away in 2005, and their mother died last May. Keeth nearly succumbed to a rare, potentially fatal blood disease, but recovered after weeks of intensive care and the constant support of his sister.

In August 2003, Australian sports scientists discovered a “specific gene linked to athletic performance,” which fueled debate over “whether top athletes can be screened and nurtured from birth,” New Scientist reported. The gene is called “alpha-actinin-3, or ACTN3,” and the researchers profiled hundreds of athletes to find the link.

The company Atlas Sports Genetics offers a $149 saliva test that analyzes ACTN3. The test is being marketed to parents of children ages 1–8 as a way to help groom their children for athletic achievement.

Oregon couple Bryan and Katie Maag are avid distance runners, as are their four children, two of whom are collegiate competitors at Princeton University, according to KPTV. The couple said if they’d had the chance, they would not have used the genetic testing to find out whether their children were born runners. "If they're talented in a certain area, it will show up sooner or later," Bryan Maag told KPTV.